Among conventional metallic gaskets of this kind, there is one which gasket described in Japanese Patent Laid-Open No. Hei 6-101761, for example. As shown in FIG. 38, this metallic gasket comprises a base plate made of a sheet of thin metal, and the base plate has a first thickness-increased portion 52 formed by fitting a grommet, for example, on the peripheral portion of the base plate defining an opening, such as a combustion chamber opening, and a rubber bead 53 formed by baking a rubber material so as to protrude from both surfaces of the base plate 50 along a seal line extending in the vicinity of the outer periphery of the base plate 50. The metallic gasket also has a second thickness-increased portion 54 formed by folding back part of the peripheral edge portion of the base plate 50, and a recoverable elastic body 55 disposed between the grommet and both surfaces of the base plate 50.
In other words, the outer peripheries of openings, such as an oil hole or a water hole, formed in the base plate 50 are sealed by surrounding the openings with a rubber bead protruding from both surfaces of the base plate 50, and the compression-deformed amount of the rubber bead 53 is regulated by the thickness-increased portions 52, 53 to thereby prevent the rubber bead 53 from being broken. To prevent. breakdown of the rubber bead 53 under compression, the compression-deformed amount of the rubber bead 53 is normally set at a maximum of about 40% or less, for example.
As another conventional metallic gasket, there is one which is described in Japanese Patent Laid-Open No. 2001-206687
As shown in FIG. 39, this metallic gasket has a first thickness-increased portion 52 formed by folding back the peripheral edge portion on the combustion chamber opening 51 side of the base plate, and inserting a shim plate 58 into the inside of the folded portion, and a second thickness-increased portion 54 formed by folding back part of the peripheral portion of an oil hole or the outer peripheral edge of the base plate, for example. In this metallic gasket, two lines of metal bead 60 in a convex-concave profile (a convex portion is formed similarly on both surfaces of the base plate 50) are formed by bending the base plate 50 in a manner to protrude on both surfaces of the base plate 50 between the first and second thickness-increased portions 52, 54, and an elastic sealing material 61 is filled in the concave portion on the reverse side of the metal bead 60. When it is desired to provide a less expensive gasket, galvanized iron is used as the material for the base plate 50.
In the former example of prior art, however, out of the thickness of the rubber bead 53, only the amount of the thickness that extends beyond the thickness-increased portions 52 and 54 is compressed and deformed to generate resilience to provide a seal. Since the rubber bead 53 needs to be formed on a flat portion so as to protrude from both surfaces of the base plate 50, the amount of rubber used to form the rubber bead 53 is limited, so that the compression-deformed amount has to be small. Consequently, the degree of demand for dimensional accuracy of the rubber bead 53 is high, the amount of concurrent deformation is small, processing of the rubber bead 53 becomes difficult, and processing accuracy becomes severe.
For example, if the thickness 0.7 t of the first thickness-increased portion 52 is designated as TO and the thickness 0.4 t of the base plate is designated as T1, the height T2 protruding to one side from the base plate is
T2 =(T0 T1) 2 + compression-deformed amount=0.15 + 0.15 = 0.4(which means the compression-deformed amount of 40% max.)=0.21
Therefore, the protruding amount of the rubber bead on one side of the base plate is 0.21 mm. Although there is a large difference between the thickness 0.7 t of the first thickness-increased portion and the thickness 0.4 t of the base plate, the rubber bead 53 has a very small amount of rubber and therefore its compression-deformed amount is small; therefore, the rubber bead 53 becomes hard to process, and the processing accuracy becomes stricter as mentioned above.
The rubber bead 53 and a recoverable elastic bodies 55 in the end face of the first thickness-increased portion 52 and the first thickness-increased portion, directly exposed to cooling water, are liable to rusting or water absorption, thus increasing chances of deterioration in performance and degeneration in quality.
As described above, when a rubber bead 53 is formed on a flat surface of the base plate 50, supposing that a metallic gasket is interposed between the joint surfaces of a cylinder head and a cylinder block, for example, a difference in thermal expansion between the cylinder head and the cylinder block gives rise to a dislocation, which causes a shearing force to be applied along the surface of the base plate 50, increasing a possibility of the rubber bead 53 separating from the base plate 50.
Since there is a gap between the base plate and each of the joint surfaces in the region from the water hole to the seal line, if cooling water other than specified is used, the base plate from the water hole to the seal line is susceptible to corrosion by the cooling water other than specified.
Further, in the region between the outer peripheral edge and the seal line (the position where the rubber bead 53 is formed) of the base plate exclusive of the folded-back portion (the second thickness-increased portion 54) of the peripheral portion of the base plate 50, because there is a gap between both side surfaces of the base plate and each of the joint surfaces of the base plate 50, assuming that the metallic gasket is used in a outboard engine, filthy water or briny water externally enters the above-mentioned gap, giving rise to corrosion of the joint surfaces of parts of cast iron or aluminum and the exposed portions of the gasket base plate 50.
Further, since cooling water directly contacts the first thickness-increased portion 52 formed by the folded-back metal and the folded-back grommet on the combustion chamber opening 51 side at high temperature, boiled water generates air bubbles inhibit the cooling effect, increasing chances of overheat. On the other hand, since cooling water directly contacts the rubber seal line, alcohol content or ethylene glycol in cooling water causes corrosion or swelling to the whole of the rubber bead 53, thus reducing durability.
In the latter example of prior art, the metallic gasket comprises a first thickness-increased portion 52 formed by folding back the whole peripheral edge on the combustion chamber side of the phase plate to seal off a high-pressure gas, and a second thickness-increased portion 54 to prevent the engine from deformation and also to protect the bead 60 (including the elastic sealing material 61) from excessive compression. Since the second thickness-increased portion 54 is formed by partly folding back the outer peripheral edge portion, in the region from the seal line by the bead 60 disposed near the outer peripheral edge of the base plate 50 or on the inner side of the bolt to the outer peripheral edge of the engine, a gap occurs on both surfaces of the base plate by an amount corresponding to the rubber thickness between the seal area joint surfaces) of the engine and the surfaces of the gasket.
Therefore, assuming that the gasket is used in an overboard motor or the like, it occurs that filthy water or briny water splashes and enters the above-mentioned gap, and because the engine radiates heat to the seal area of the engine of cast iron or aluminum and the exposed portions of the gasket base plate 50, the adhesion of briny water or the like causes corrosion.
As the engines which use the above-mentioned metallic gasket are reduced in size and weight, the deck width for the seal area of the engine becomes narrower, and it becomes difficult to secure a space for forming a wide metal bead 60 in a wide, undulated structure protruding on both surfaces of the base plate 50 in addition to the width of the folded-back portion along the peripheral edge of the combustion chamber opening. This can be said of the region around the openings, such as bolt holes, water holes and oil holes. As the engines that uses metallic gaskets become smaller and lighter, a problem is that it becomes difficult to adopt the metallic gasket structure of the prior art (the latter example).
Since the elastic sealing material 61 is used only in the concave portion in contrast to the present patent application, if the hardness of the base plate is low, when the elastic sealing material 61 is deformed in compression, the metal bead 60 is likely to deform in a manner to warp in the through-thickness direction, and when this deformation occurs, this leads to a decrease in sealing pressure by a amount corresponding to the deformation.
Further, another example of the conventional metallic gasket is disclosed in Japanese Patent Laid-Open 2001-173791, for example.
As shown in FIG. 40, this metallic gasket comprises two base plates 50.
More specifically, out of the two base plates 50, a thickness-increased portion 52 is formed at the edge portion on the combustion chamber opening 51 side of a thicker base plate (upper base plate). A base-plate bead 53 of a convex structure is formed on each of the two base plates 50 in a position on the outer side of the thickness-increased portion 52 and at a height higher than the thickness-increased portion 52. The metallic gasket is composed by stacking two base plates 50 in such a way that the convex portion sides of the two base-plate beads 53 face each other. Further, the concave portions facing outside are filled with an elastic sealing material 54.
When the metallic gasket is disposed between opposing joint surfaces of a cylinder block and a cylinder head and fastened with clamping bolts, the base-plate beads 53 are compressed and deformed down to the thickness of the thickness-increased portion around the inner peripheral edge of the combustion chamber opening and simultaneously the elastic sealing material 54 filled in the concave portion is compressed and deformed to seal a combustion gas, oil, and cooling water pressures by a sealing pressure from a composite spring made up of the spring force of the base-plate bead 53 and the spring force of the elastic sealing material 54. Needless to say, there are conventional metallic gaskets without any elastic sealing material 53 filled in the concave portion and also there are conventional metallic beads made up of a single piece of base plate.
In this conventional metallic gasket (Refer to FIG. 40), however, the base-plate bead 53 and the elastic sealing material 54, when deformed by fastening, jointly generate a resilience to apply a required sealing pressure along the seal line.
However, when the base plate 50 is formed by metal plate of low hardness to prevent fatigue failure and reduce cost of the base plate 53, in the above metallic gasket, when bolts are fastened and the elastic sealing material 54 of the concave portion of the bead is compressed and deformed, an external force is applied such that the base plates 50 and the base-plate beads 53 are compressed and deformed in a manner to warp in the through-thickness direction. Since the base plates 50 are formed of metal of low hardness as mentioned above, the beads have a low shape-retaining force and accordingly the base plate 50 have an insufficient deformation-preventive force and hence a low sealing property.
By repeated load by repetition of operation and stoppage of the engine, after a long period of use, problems arise, such as a decrease in axial tension of the clamping bolts, changes with time of the base-plate bead 53 on the base plate 50, or deterioration in the elastic sealing material 54 of the concave portion of the bead; therefore, the sealing surface pressure is likely to drop. Such problems tend to occur particularly at overhanging parts on the outer side of the clamping bolts.
When the elastic sealing material 54 is formed by baking in the concave portions of the base plates, even if the elastic sealing material 54 at high temperature is filled in the concave portions, it changes in volume by an amount of thermal expansion during subsequent cooling and being left open, the center portion of the elastic sealing material 54 where the thickness is at its highest contracts by an amount of thermal shrinkage. This is disadvantageous when the surface pressure drops as described above. Such a phenomenon as this seems to be likely to occur particularly when the gasket is mounted in the engine which has been assembled with a weak fastening axial tension.